/*
 *
 * (c) Copyright Ascensio System Limited 2010-2018
 *
 * This program is freeware. You can redistribute it and/or modify it under the terms of the GNU 
 * General Public License (GPL) version 3 as published by the Free Software Foundation (https://www.gnu.org/copyleft/gpl.html). 
 * In accordance with Section 7(a) of the GNU GPL its Section 15 shall be amended to the effect that 
 * Ascensio System SIA expressly excludes the warranty of non-infringement of any third-party rights.
 *
 * THIS PROGRAM IS DISTRIBUTED WITHOUT ANY WARRANTY; WITHOUT EVEN THE IMPLIED WARRANTY OF MERCHANTABILITY OR
 * FITNESS FOR A PARTICULAR PURPOSE. For more details, see GNU GPL at https://www.gnu.org/copyleft/gpl.html
 *
 * You can contact Ascensio System SIA by email at sales@onlyoffice.com
 *
 * The interactive user interfaces in modified source and object code versions of ONLYOFFICE must display 
 * Appropriate Legal Notices, as required under Section 5 of the GNU GPL version 3.
 *
 * Pursuant to Section 7 § 3(b) of the GNU GPL you must retain the original ONLYOFFICE logo which contains 
 * relevant author attributions when distributing the software. If the display of the logo in its graphic 
 * form is not reasonably feasible for technical reasons, you must include the words "Powered by ONLYOFFICE" 
 * in every copy of the program you distribute. 
 * Pursuant to Section 7 § 3(e) we decline to grant you any rights under trademark law for use of our trademarks.
 *
*/


using System;

namespace ASC.Xmpp.Core.IO.Compression.Checksums
{

    #region usings

    #endregion

    /// <summary>
    ///   Computes Adler32 checksum for a stream of data. An Adler32 checksum is not as reliable as a CRC32 checksum, but a lot faster to compute. The specification for Adler32 may be found in RFC 1950. ZLIB Compressed Data Format Specification version 3.3) From that document: "ADLER32 (Adler-32 checksum) This contains a checksum value of the uncompressed data (excluding any dictionary data) computed according to Adler-32 algorithm. This algorithm is a 32-bit extension and improvement of the Fletcher algorithm, used in the ITU-T X.224 / ISO 8073 standard. Adler-32 is composed of two sums accumulated per byte: s1 is the sum of all bytes, s2 is the sum of all s1 values. Both sums are done modulo 65521. s1 is initialized to 1, s2 to zero. The Adler-32 checksum is stored as s2*65536 + s1 in most- significant-byte first (network) order." "8.2. The Adler-32 algorithm The Adler-32 algorithm is much faster than the CRC32 algorithm yet still provides an extremely low probability of undetected errors. The modulo on unsigned long accumulators can be delayed for 5552 bytes, so the modulo operation time is negligible. If the bytes are a, b, c, the second sum is 3a + 2b + c + 3, and so is position and order sensitive, unlike the first sum, which is just a checksum. That 65521 is prime is important to avoid a possible large class of two-byte errors that leave the check unchanged. (The Fletcher checksum uses 255, which is not prime and which also makes the Fletcher check insensitive to single byte changes 0 - 255.) The sum s1 is initialized to 1 instead of zero to make the length of the sequence part of s2, so that the length does not have to be checked separately. (Any sequence of zeroes has a Fletcher checksum of zero.)"
    /// </summary>
    /// <see cref="ICSharpCode.SharpZipLib.Zip.Compression.Streams.InflaterInputStream" />
    /// <see cref="ICSharpCode.SharpZipLib.Zip.Compression.Streams.DeflaterOutputStream" />
    public sealed class Adler32 : IChecksum
    {
        #region Members

        /// <summary>
        ///   largest prime smaller than 65536
        /// </summary>
        private static readonly uint BASE = 65521;

        /// <summary>
        /// </summary>
        private uint checksum;

        #endregion

        #region Constructor

        /// <summary>
        ///   Creates a new instance of the Adler32 class. The checksum starts off with a value of 1.
        /// </summary>
        public Adler32()
        {
            Reset();
        }

        #endregion

        #region Methods

        /// <summary>
        ///   Resets the Adler32 checksum to the initial value.
        /// </summary>
        public void Reset()
        {
            checksum = 1;
        }

        /// <summary>
        ///   Updates the checksum with the byte b.
        /// </summary>
        /// <param name="bval"> The data value to add. The high byte of the int is ignored. </param>
        public void Update(int bval)
        {
            // We could make a length 1 byte array and call update again, but I
            // would rather not have that overhead
            uint s1 = checksum & 0xFFFF;
            uint s2 = checksum >> 16;

            s1 = (s1 + ((uint) bval & 0xFF))%BASE;
            s2 = (s1 + s2)%BASE;

            checksum = (s2 << 16) + s1;
        }

        /// <summary>
        ///   Updates the checksum with an array of bytes.
        /// </summary>
        /// <param name="buffer"> The source of the data to update with. </param>
        public void Update(byte[] buffer)
        {
            Update(buffer, 0, buffer.Length);
        }

        /// <summary>
        ///   Updates the checksum with the bytes taken from the array.
        /// </summary>
        /// <param name="buf"> an array of bytes </param>
        /// <param name="off"> the start of the data used for this update </param>
        /// <param name="len"> the number of bytes to use for this update </param>
        public void Update(byte[] buf, int off, int len)
        {
            if (buf == null)
            {
                throw new ArgumentNullException("buf");
            }

            if (off < 0 || len < 0 || off + len > buf.Length)
            {
                throw new ArgumentOutOfRangeException();
            }

            // (By Per Bothner)
            uint s1 = checksum & 0xFFFF;
            uint s2 = checksum >> 16;

            while (len > 0)
            {
                // We can defer the modulo operation:
                // s1 maximally grows from 65521 to 65521 + 255 * 3800
                // s2 maximally grows by 3800 * median(s1) = 2090079800 < 2^31
                int n = 3800;
                if (n > len)
                {
                    n = len;
                }

                len -= n;
                while (--n >= 0)
                {
                    s1 = s1 + (uint) (buf[off++] & 0xFF);
                    s2 = s2 + s1;
                }

                s1 %= BASE;
                s2 %= BASE;
            }

            checksum = (s2 << 16) | s1;
        }

        #endregion

        #region IChecksum Members

        /// <summary>
        ///   Returns the Adler32 data checksum computed so far.
        /// </summary>
        public long Value
        {
            get { return checksum; }
        }

        #endregion
    }
}